MediaModelFactory.findPart moved to SmilHelper for use in both places.
findPart updated to include looking in ContentId even if the "src" does not have the cid: prefix. Noticed from a Stock GS4.
SmilHelper now has a fixup(...) method which removes duplciate parts.

Change-Id: I1e1b6b0792888797123fadd795dd33b141ce8ea2